LTC3410B_15 Datasheet, PDF (9/16 Pages) Linear Technology – 2.25MHz, 300mA Synchronous Step-Down Regulator in SC70

LTC3410B
APPLICATIO S I FOR ATIO
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that the capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor, or choose a capacitor rated at a higher
temperature than required. Always consult the manufac-
turer if there is any question.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C_OUT has been met, the RMS current rating
generally far exceeds the I_RIPPLE(P-P) requirement. The
output ripple ∆V_OUT is determined by:
∆V_OUT
≅
∆I_L
⎛
⎝⎜
ESR
+
1⎞
8fC_OUT ⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ∆I_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ∆I_L increases with input voltage.
If tantalum capacitors are used, it is critical that the
capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface mount tantalum. These are specially constructed
and tested for low ESR so they give the lowest ESR for a
given volume. Other capacitor types include Sanyo
POSCAP, Kemet T510 and T495 series, and Sprague 593D
and 595D series. Consult the manufacturer for other
specific recommendations.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
LTC3410B’s control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, V_IN. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at V_IN, large enough
to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Output Voltage Programming (LTC3410B Only)
The output voltage is set by a resistive divider according
to the following formula:
V_OUT = 0.8V^{⎛⎝⎜}1+ ^{RR21⎞⎠⎟}
(2)
The external resistive divider is connected to the output,
allowing remote voltage sensing as shown in Figure 2.
V_FB
LTC3410B
GND
0.8V ≤ V_OUT ≤ 5.5V
R2
R1
3410 F02
Figure 2. Setting the LTC3410B Output Voltage
3410bfa
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